1. Field of the Invention
The present invention relates to a double-stage phase-diversity receiver for use in not only coherent optical fiber communications employing an optical fiber but also, electrical communications, and radio wave communications and light wave communications that use spatial propagation.
2. Description of the Related Art
Receivers used in coherent optical fiber communications are basically classified into two schemes, a heterodyne scheme and a homodyne scheme. In the heterodyne scheme with a very high speed of several Gbits/sec, the intermediate frequency (IF) becomes 10 to 20 GHz, which makes it difficult to realize high-performance receivers due to restriction on the frequency response characteristic of a photodetector or microwave circuit technology. In the homodyne scheme, by way of contrast, although the light source is required to have a narrow spectral width, the above difficulty can be avoided because the optical signal is converted into a baseband signal. In this respect, research on this homodyne scheme has recently been accelerated with improvement of light sources. Further, attention has been paid to a phase-diversity scheme in which the requirements for optical phase stability on laser diodes for use in a transmitter and/or for use in a local oscillator in a receiver are much relaxed. In this scheme, as in well as older homodyne schemes, however, (a) it is not possible to compensate in the receiver the delay distortion produced by group delay of optical fibers, while this compensation is possible in the heterodyne scheme, and (b) it is technically more difficult to realize coherent ASK (Amplitude Shift Keying) or PSK (Phase Shift Keying) demodulators in baseband than in intermediate frequency (IF) range. Of these intrinsic limitations to performance of the phase-diversity scheme, the limitation (b) can be overcome, as recently proposed, by converting baseband signal into an intermediate frequency, before demodulation. Yet no solution has yet been proposed to the first limitation (a).
Silica fiber has the lowest transmission loss in 1.55-.mu.m wavelength band, but in this wavelength band, a relatively large group delay distortion occurs in the signal waveform because of wavelength dispersion. This group delay distortion restricts the transmission speed or the transmission distance particularly in signal transmission at a high speed of several Gbits/sec. As a solution to this shortcoming, dispersion-shifted fibers or dispersion-flattened fibers may be employed to reduce the wavelength dispersion. These optical fibers, however, have higher transmission loss; the former type fibers have a narrow region where the dispersion is negligible and the latter fibers are difficult to manufacture.